The present invention relates to imaging, and more particularly, to a four-color mosaic tiling pattern and color interpolation.
A simplified cross sectional view of an imaging system is illustrated in FIG. 1. Optical system 102 focuses electromagnetic radiation onto a focal plane, which may be taken as Color Filter Array (CFA) 104 and pixel sensor array 106. The CFA is usually deposited over pixel sensor array 106 by photo-lithographic techniques well known to the semiconductor industry. Pixel sensor array 106 is an array of pixel sensors, where in general, a pixel sensor is any sensor which absorbs radiation and provides a signal indicative of the absorbed radiation. Pixel sensor array 106 may be, for example, an array of charge coupled devices (CCD), or an integrated array of CMOS (Complementary Metal Oxide Semiconductor) pixel circuits. A pixel circuit may comprise a photo diode, where photons absorbed by the photo diode generate electron-hole pairs, along with additional circuits to provide an electrical signal, either a voltage or current signal, indicative of the number of photons absorbed by the photo diode. Photons incident upon various pixel circuits are pictorially indicated by 108.
The spectral content of electromagnetic radiation focused onto a focal plane depends upon, among other things, the imaged subject, the illumination of the subject, the transmission characteristics of the propagation path between the imaged subject and optical system 102, the materials used in optical system 102, as well the geometric shape and size of optical system 102. For consumer imaging systems, the spectral range of interest is the visible region of the electromagnetic spectrum.
The CFA is an array of filters, usually contiguous and deposited over pixel sensor array 106 so that each pixel sensor is substantially sensitive to only the electromagnetic radiation passed by one filter. (A filter in the CFA may actually be a composite filter manufactured from two or more filters, so that the transfer function of the resulting filter is the product of the transfer functions of its constituent filters.) Each filter in the CFA passes electromagnetic radiation within some spectral range. For example, a CFA may be composed of red, green, and blue filters, so that the pixel sensors may provide signals indicative of the visible color spectrum.
If there is not an infrared blocking element somewhere in the optical chain, then infrared radiation (IR), typically considered to be light with a wavelength longer than 780 nm, may also be focused upon the focal plane. Imaging sensors or devices based on silicon technology typically require the use of infrared blocking elements to prevent IR from entering the imaging array. Silicon-based devices will typically be sensitive to light with wavelengths up to approximately 1200 nm. If the IR is permitted to enter the array, the device will respond to the IR, and generate an output image signal. Since the purpose of an imaging system (in this context) is to create a representation of the visible light present in a scene, the IR may introduce a false response and distort the image. In a monochrome (black and white) imaging system, the result may be an obviously distorted rendition. For example, foliage and human skin tones may appear unusually light. In a color imaging system, the introduction of IR may distort the coloration and produce an image with incorrect and de-saturated color.
A common method for preventing these difficulties is to use ionically colored glass or a thin-film optical coating on glass to create an optical element which passes visible light (typically from 380 nm to 780 nm) and blocks the IR. This element can be placed in front of the taking lens, located within the lens system, or it can be incorporated into the imager package. The principle disadvantages to this approach are cost and added system complexity.
An alternative approach is to allow IR to enter the focal plane, and then remove the IR signal electronically. In this approach, the CFA will have some IR pass filters (which also substantially block visible light), and the pixel sensor array would have some pixel sensors responsive to IR only. The effect of IR upon an image signal is substantially reduced by electronically subtracting signals generated by IR pixel sensors from signals generated by pixel sensors responsive to both IR and visible light.
It is therefore desirable to provide for a CFA with a suitable arrangement of IR and visible color filters, and to provide for a method of processing the resulting pixel signals to obtain proper color interpolation without the need of an IR blocking filter.